In a water treatment field such as sewage treatment, there has been widely used an activated sludge method in which microorganisms are made to proliferate to metabolize and decompose organic matter in sewage. In water purifying treatment of the sewage by the activated sludge method, sludge flocs, in which the microorganisms and/or organic polymers are thickened in a very dense state, are formed. Sludge generated by excessive proliferation of the microorganisms is drawn from a reaction tank as excess sludge to become industrial wastes. The amount of residual of a sludge disposal field is depleted and disposal cost is soared; and accordingly, a reduction in the generation amount of sludge is needed. At present, there has been studied a reduction in sludge volume by means of solubilization treatment of solid components of the sludge and/or anaerobic digestion treatment of the sludge by chemical treatment that uses ozone and/or chemicals (for example, Patent Documents 1 to 5).
On the other hand, depletion of phosphate ore is apprehended and therefore reuse of phosphorus resources is emphasized. In a sewage treatment plant, as countermeasures against red tides and blue-green algae, phosphorus in sewage treated water is made to accumulate in the microorganisms and the phosphorus is removed from the sewage treated water as sewage sludge. Accordingly, a relatively high concentration of phosphorus is contained in the sewage sludge. If the anaerobic digestion treatment of the sludge is performed in order to reduce the generation amount of sludge, the phosphorus is eluted again from the excess sludge to form insoluble magnesium and phosphate (MAP) and to become scales. The scales block piping of an anaerobic digestion tank and accordingly it causes an impediment to the maintenance of facilities.
With the aim of a reduction in generation amount of sludge, there is proposed a method of performing solubilization treatment of the sludge by chemical treatment. In a treatment method of dissolving the sludge by using ozone-containing gas (hereinafter, referred to as “ozone gas”) and alkali, the sludge is exposed to the ozone gas and then alkali is added to perform solubilization of the sludge solid components; and in the latter stage, phosphorus recovery treatment or anaerobic digestion treatment is performed to reduce the generation amount of sludge (e.g. Patent Document 1). Furthermore, after the ozone gas is mixed with the sludge, there is proposed a method of performing solubilization treatment of the sludge in a depressurized reaction tank (e.g. Patent Document 2). Further, there is proposed a method in which the ozone gas is mixed with the sludge by using an ejector to perform solubilization treatment of the sludge (e.g. Patent Document 3).